In PowerShell, the variable "$_" has special importance. It works like a placeholder in scenarios like this one:
PS> Get-ChildItem $env:windir | Where-Object { $_.Length -gt 1MB }
In PowerShell v3, there is an alias for the cryptic "$_": $PSItem. So now code can become more descriptive:
PS> Get-ChildItem $env:windir | Where-Object { $PSItem.Length -gt 1MB }
Then again, in PowerShell v3, "$_" isn't necessary in many scenarios anymore at all. You could also write:
Get-ChildItem $env:windir | Where-Object Length -gt 1MB